Avoid direct access to IO internals.

[ioquatix]

Fixes https://github.com/socketry/nio4r/issues/299.

Types of Changes

• Bug fix.
• Maintenance.

Contribution

• [x] I added tests for my changes.
• [x] I tested my changes locally.
• [x] I agree to the Developer's Certificate of Origin 1.1.

[MSP-Greg]

To clarify, ruby/ruby head/master made ABI changes to IO (see https://github.com/ruby/ruby/commit/18e55fc1e1ec), which caused CI build failures with several extension gems. This was mentioned in #299.

The changes were reverted in https://github.com/ruby/ruby/commit/85dcc4866d9f. But, the head builds used in GitHub Actions setup-ruby are only updated daily, and the builds used for the CI in this PR do not contain the 'reversion' commit.

Hence, this PR works with both the updated IO ABI and the previous ABI, which used in Ruby 3.2.